CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart".
We all would have seen lot of sites which show an image which is disturbed and has some text in it and we need to read and enter that text in to a textbox so that we can avoid automatic spam bots or automated testing tools entering the page.
This is a project of Carnegie Mellon university and they hold the copyrights for it.
http://en.wikipedia.org/wiki/Captcha
http://www.aladdin.cs.cmu.edu/
ALADDIN is the algorithmic adaptation dissemination and integration team at CMU their papers are great :)
